A conventional aquarium for keeping fish, especially tropical fish, for enjoyment may have its water deteriorated by poisonous substance such as nitrate and nitrite produced by rotten excessive food and excessive fish waste. Although it is a known fact the nitric bacteria can perform nitrification to get rid of nitrogen, nitrate, nitrite, in water. But if the water in an conventional aquarium should contain excessive poisonous substance such as nitrate, nitrite for nitric bacteria therein to consume, fish therein would get disease, and die if worst, to a resultant loss to a grower. Some growers often put nitric bacteria in an awuarium to promote nitrification so as to keep balance of biological environment therein, and this traditional method may be a little effective in keeping a neutral balance of nitrate, nitrite, and nitrogen. Nevertheless, nitric bacteria may fasten on glass walls of an aquarium smearing them, and then it is hard to look through the glass to enjoy watching at fish therein.